The correct answer is B. Known-plaintext attack because the scenario explicitly provides paired samples of plaintext and ciphertext for the same underlying data. In a known-plaintext attack, the analyst possesses one or more examples where the original message (plaintext) is known and the corresponding encrypted output (ciphertext) is also available. These pairs can be used to analyze the cipher’s behavior, validate hypotheses about modes/parameters, and—depending on the algorithm, implementation weaknesses, and key management—attempt to recover the encryption key or decrypt other ciphertexts encrypted under the same key.
Here, Evelyn has “original template records (standard headers and form fields)” and their “encrypted counterparts” in the stolen backup set. Medical record formats commonly contain predictable structures (fixed headers, repeated field names, standardized forms), which increases the likelihood of having accurate known plaintext segments. With enough known plaintext/ciphertext pairs, an attacker may identify patterns caused by weak encryption choices, reused keys, reused IVs/nonces, insecure modes (e.g., ECB revealing structure), or flawed custom crypto. Even when strong algorithms are used, known-plaintext material can still be valuable for confirming the encryption scheme and detecting implementation errors.
Why the other options are not correct: Ciphertext-only assumes the attacker has only encrypted data and no plaintext examples—contradicted by the templates. Chosen-plaintext requires the ability to submit arbitrary plaintexts to an encryption oracle and receive ciphertexts, which the scenario does not indicate. Chosen-ciphertext requires the ability to submit ciphertexts to a decryption oracle and observe outputs, also not described.
Because Evelyn has real, matching plaintext-ciphertext pairs from the same dataset, the most appropriate cryptanalytic approach is a known-plaintext attack.
